This invention relates to pre-expanded cation exchange membranes, processes for making such membranes, electrochemical cells utilizing such membranes, and methods for preparing said cells for use in chloralkali processes.
Ion exchange membranes formed from fluorinated polymers having carboxylic acid and/or sulfonic acid functional groups are known in the art. A principal use of such membranes is in chloralkali electrolysis cells. Ion exchange membranes tend to swell when they come into contact with cell electrolytes, resulting in wrinkling of the membranes and poor cell performance. Accordingly, it is desirable to install the membrane in an expanded condition which will revert to a taut state when the cell operating liquors are present.
A number of approaches for pre-expanding membranes have been suggested in the art. Japanese Patent Application No. 52-72398, published on June 16, 1977, discloses that chloralkali cell membranes can be swelled prior to installation in the cell by immersing them in a "hydrophilic solvent," with or without added water. Suggested solvents are alcohols, aldehydes, ketones, acids, esters, amides, amines and nitriles, and specific examples to isopropyl/alcohol, ethylene glycol, dimethylacetamide, tetrahydrofuran and ethanol are provided. In each instance, the membrane, after being immersed in the solvent for a period of time, was wiped dry and immediately placed in the chloralkali cell. There was no indication that the pre-swelled membrane could be stored for any period of time.
Another approach is described in Research Disclosure, February 1982, pp 32-34. Here it is suggested that the membrane be swelled with water, brine, caustic, lower alcohols, glycols or mixtures thereof.
U.S. Pat. No. 4,376,030, issued to Ezzell et al. on Mar. 8, 1983, discloses a method for providing fluoropolymer ion-exchange membranes with dimensional stability by treating the membranes with at least one liquid amine to form amine salts, then drying and storing the membranes.
U.S. Pat. No. 4,270,996, issued to Suhara et al. on June 2, 1981, discloses that wrinkling of fluorinated polymer membranes can be alleviated by treating the membranes with a base or salt to obtain a specified weight gain which depends on ion exchange capacity (IEC). The weight gains range from 2% at IEC 0.5 to 50% at IEC 2.0.
Japanese Patent Application No. 58-163446, published on Sept. 28, 1983, discloses swelling unreinforced membranes to a linear magnification of more than 1.1, then restraining and drying. The swelling agent may be monohydricalcohols such as methanol, ethanol, propanol and butanol, dihydric alcohols such as ethylene glycol and propylene glycol, trihydric alcohols such as glycerine, ethers sucn as methyl and ethyl ethers, and nitriles such as acetonitrile.
Another publication of possible relevance is Japanese Patent Application No. 51-73991, published on June 26, 1976. This application does not speak in terms of "pre-swelling" cation exchange membranes but rather in terms of improving the cation-to-anion selectivity of the membranes. This is done by incorporating in the molecule, in the interior, and/or on the surface of the fluorocarbon membrane a polymer ----CX.sub.2 CXY--.sub.n O--.sub.m, where X-H orhalogen, Y=H, halogen, CH.sub.3 or its halogen substitution product, n=1 or 2 and m=an integer of 2 or higher.
Despite the various methods described above for pre-swelling or otherwise improving fluorocarbon cation exchange membranes, the common practice is still to presoak the membrane in dilute solutions of salts or bases (e.g., 2% NaHCO.sub.3 or NaOH) prior to installation in the electrolytic cell. Precautions must be taken to insure that the membranes do not dry out before or after installation in the cell. Thus, the membrane must be immediately installed in the cell after expansion is complete and, after installation, the cell must be kept wet. This is disadvantageous as it involves an additional, complicated process step at the point of cell assembly. Another disadvantage is that the wet membranes may cause corrosion of the electrodes, especially iron cathodes, before cell start-up.